The present invention relates to a chemically sensitive element, and more particularly, to such an element which may be used to determine the activity or the concentration of a specified ion contained in an electrolyte.
There has been proposed a chemically sensitive element for determining the activity or the concentration of a specific ion contained in an electrolyte, which element has the construction of the field-effect transistor type as shown in FIG. 1, which is also known as an ion sensor. The ion sensor 11 of the field-effect transistor (hereinafter abbreviated as ISFET), which is oblong as viewed in a plane, comprises a gate 12, which has an ion sensing portion at the right end of FIG. 1, a lead portion 13 for a source and a lead portion 14 for a drain at the middle of the left side of FIG. 1.
In a section view of the gate 12, taken on section line II--II in FIG. 2, its configuration is long sideways hexagonal in shape and there are provided an n-type drain diffusion region 26 at the middle and upper surface side of a p-type silicon substrate 22 and an n-type source diffusion region 27 at both ends thereof. The entirety of the gate is covered by a gate insulating membrane 23 and is further covered by a protecting membrane 24 such that when immersed in a solution to be examined the gate insulating membrane 23 will not swell due to the solution. In addition, an ion sensing membrane 25 is provided on the upper surface of the protecting membrane 23 by the drain and the source diffusion region 26 and 27 sides.
The ion sensing membrane 25 varies in its composition depending upon the specific ion to be sensed. By way of example, when a sensing membrane for sensing a hydrogen ion (H.sup.+) is provided, it shows a potential gradient of approximately 50 mV/pH in gate voltage.
However, when light impinges on membrane 25 the light reaches the silicon substrate 22, resulting in that an erroneous signal to the extent of 0.1 in pH, even in indoor illumination, due to the photoelectric effect within the semiconductor. Thus it is impossible to achieve an accurate measurement of the concentration of an ion.